In the excavation, transportation and storage of oil, it is frequently found that various pipes, machines and tanks made of steel are corroded and corrosion-embrittled by sulfides, especially, hydrogen sulfide (H.sub.2 S) contained in oil. Also, welded portions of tanks for storing liquidized propane gas (LPG) are frequently corrosion-embrittled by sulfides, especially, hydrogen sulfide.
Recently, the steel material used to produce the above-mentioned pipes and tanks is required to have an increased mechanical strength. However, it is known that the increase in the mechanical strength of the steel material causes the resistance of the steel material to sulfide-corrosion cracking to be deteriorated. Also, hydrogen sulfide is highly corrosive to such steel material. Therefore, when the pipes or tanks are kept in contact with oil or LPG containing hydrogen sulfide for a long period of time, the pipes and tanks are corroded so that the thicknesses of the peripheral walls of the pipes and tanks are decreased. The reduced thickness of the walls of the pipes and tanks causes them to exhibit a poor mechanical strength so that they cannot satisfactorily effect their functions.
In the past, many attempts were made to resolve the above-mentioned problem. However, none of the attempts were successful in providing a high tensile low alloyed steel having excellent resistances to both sulfide corrosion cracking and corrosion.
Generally, it has been believed that, in order to provide a certain level of resistance to sulfide corrosion cracking, the mechanical strength of the steel material should be limited to a certain range. That is, it has been considered that a lower limit of the resistance of the steel material to sulfide corrosion cracking can be set by determining an upper limit of the mechanical strength of the steel material.
Also, it has been believed that, in order to increase the resistance of the steel material to sulfide corrosion cracking, satisfactory quenching and tempering procedures should be applied to the steel material so that the steel material has a tempered martensite structure.
The above-mentioned beliefs are accepted in API Standard, 5AC, relating to pipes for oil-wells. That is, the Standard stipulates an upper limit in the hardness of the steel material, and states that quenching and tempering operations should be applied to the steel material. Also, the Standard stipulates the lower limit in the tempering temperature to be applied to the steel material. However, even if a steel material is produced in accordance with the Standard, the resultant steel material usually exhibits an unsatisfactory resistance to sulfide corrosion cracking.
For the purpose of protecting the low alloyed steel material from corrosion, usually, the steel material is coated with a corrosion-resistant paint or protected by means of a cathodic protection or by applying a corrosion-inhibitor to the corrosive environment. However, substantially no attempts have been made to increase the resistance of the steel material itself to the corrosion.